1. Field of the Invention:
This invention relates to apparatus and techniques for improving the impact resistance of relatively thin ophthalmic lenses and more particularly to improving the impact resistance of thin ophthalmic lenses carried within an eyeglass frame and in particular those lenses which have been chemically strengthened.
2. Brief Description of the Prior Art:
In approximately 1972 the U.S. Government identified a need for regulating the strength characteristics of ophthalmic lenses. It adopted standards, which at that time were already in use in the industry by many reputable lens manufacturers, ophthalmic laboratories and practitioners diagnosing, prescribing and filling patient requirements for opthalmic eyeglass wear. The standards in use are included in the American National Standard Requirements for First Quality Prescription Ophthalmic Lenses (ANSI Z 80.1) and presently ophthalmic eyeglass lenses are manufactured to meet those standards.
In addition, manufacturers have always of necessity been concerned about the yield of their manufacturing processes and the quality of their product. Numerous efforts have been made to minimize and control the amount of breakage and damage in the manufacture and subsequent assembly of lenses to eyeglass frames. Improved handling techniques have been incorporated, as have changes to either the eyeglass lens or the frame in which it is being incorporated in order to improve the yield and the process of filling customers' prescriptions and needs. All this has been done while still attending to the consumer's style and fashion demands.
Efforts in these directions to address the improved quality question are evident, for example, from the disclosures in the following discussed patents. In U.S. Pat. No. 1,177,367, issued in 1916, it is disclosed that non-metallic rims, having popularity at that time, were susceptible to fracture under shock and strain. Therefore, consideration was given to provide a metal support on the outside of non-metallic material used in mounting lenses. In that particular case, the concern was for providing strength to the non-metallic part of the frame and not the lens. Likewise, in U.S. Pat. No. 1,241,716, issued in 1917, similar considerations were given but with respect to facilitating assembly of the lens to or separation from the non-metallic materials. in 1929, U.S. Pat. No. 1,704,232 identified a cushion incorporated to protect the lenses, when the frame is dropped. In this particular situation the problem being addressed is one of shock to the lens through a collateral impact provided upon the frame or ophthalmic mounting.
In 1930 U.S. Pat. No. 1,785,929 addressed a similar problem and pointed out the importance of providing a supporting metallic frame when using a non-metallic rim included for ornamental appearance. There the combination of the two provided for marketing of the frame to practitioners without any lenses being included. The structural combination provided a unitary piece. In 1935 concern over difficulties in loose lenses in the frame was addressed by the industry. This problem was discussed in U.S. Pat. No. 1,995,617 wherein a lining made of cellulose composition in the form of a strip gasket was utilized. The gasket was shaped with a cross-section to fit the lens edge and inherently curved to encompass the len's outer periphery. This concept allegedly had the flexibility of being used for different size lenses and shapes.
In 1940 U.S. Pat. No. 2,221,736 addressed the problem of mounting lenses in a rim, such that differences in sizes of the rim due to manufacturing inconsistencies or expansion or contraction because of heat changes will be readily absorbed and not transferred to a lens within the rim. The specification stated that the lens when bound and in position in the rim may break by reason of such changes in size and pressure developed by contraction and expansion. Disclosed is a plano flat lens having a uniform V-shape edge formation for the application of a rubber latex material thereto prior to the lens being mounted within the metal rim. It is presumed that natural rubber latex was under consideration. This presumption is based on, for example, G. R. Vila's statement in his article entitled "Types of Latex" at page 263 of The Vanderbilt Rubber Handbook, Edited by S. S. Rogers, Ninth Edition 1948, published by R. T. Vanderbilt Co., New York, New York, wherein he states: "Prior to World War II the latex technologist was limited for the most part to one material--Hevea latex-- either normal, centrifugal or creamed. Today, in addition to Hevea, a variety of synthetics are available, . . . ". The patent describes tightening of the lens by means of the end pieces such that there is a yieldingly bind of the lens when in position by the lens edge coating. In this way, contraction and expansion is suggested to be accommodated by reason of the coating.
Circa 1945 ophthalmic eyeglass lenses and frames that included only partial encompassing of the lens were popular. However, the lenses were still subject to breakage and one solution for lessening such breakage was disclosed in U.S. Pat. No. 2,387,789 wherein a band of plastic material, suggested to be invisible, was incorporated about the contour edges of the lens which also thereby protected the lens against chipping should the lens be struck at its edge.
With the advent of more modern eyeglass frame configurations the mounting of the eyeglass lenses into such frames became more difficult. Therefore, in order to more positively secure lenses in frames, techniques such as those in U.S. Pat. No. 2,914,986 were employed. In that patent a gasket material, for example, was incorporated into a metal frame to improve the seating of the lens regardless of the outside peripheral contour of a lens defined by concave and convex sides and regardless of the irregularities or inaccuracies in the periphery edge cross section of the lens after being ground to mount within the frame. In addition, the resilient liner allegedly provided for stable retention of the lens in the frame regardless of shock and vibration to which the frame might be subjected. Further, the liner within the metal mounting compensated for unequal compression caused by improper edging of the lens or misfitting of the lens to the mounting. This patent suggested that its inventive contribution minimized lens breakage during the lens insertion and fitting operation.
The concern for improving strength capabilities of the combination frame and lens has continued into the present era as illustrated, for example, in U.K. Patent Specification 1,566,876 published May 8, 1980. Here chemically strengthened eyeglass lenses were being subsequently cut and edged, followed by mounting into ophthalmic frames. The disclosed process provided for preventing a significant decrease in the impact resistance of the unmounted lens due to the removal of the compression layer by edging subsequent to the chemically strengthening. This was accomplished by providing a polymer coating, subsequently polymerized, on the edge surface. This coating is alleged to compensate for the decrease in impact resistance which occurs during the cutting and edging of such a chemically strengthened lens. Further, myoptic rings which occur in strong minus prescriptions for such lenses are reduced.
The glass industry most recently has embarked on an era of new glass materials for the manufacture of eyeglass lenses which materials have demonstrated many improved optical and cosmetic qualities including having photochromic properties readily suitable for gradient tinting and further these glasses combined with strengthening by chemical tempering are suitable for making thinner lenses. Natural concern has arisen over the relative strength of such materials particularly when made in the form of eyeglass lenses. To evaluate these properties, tests have been conducted utilizing equipment standard in the industry and manufactured in accordance with ANSI Z 87.1-1968 entitled U.S.A. Standard Practice for Occupational and Educational Eye and Face Protection, dated Sept. 18, 1968 and the FDA Statement of Policy, Section 3.84 entitled Use of Impact Resistance Lenses in Eyeglasses and Sunglasses, as printed in the Federal Register of Feb. 2, 1972.
As a standard to make a comparison during the development of this invention, 2.2 millimeter thick air tempered strengthened lenses were compared with 1.6 millimeter thick chemically tempered strengthened lenses. The lenses of both thicknesses were of similar material and shape, which shape is commonly known in the industry as an aviator shaped lens. Both groups of lenses had passed the 50 inch drop ball test according to the FDA Policy Statement of Feb. 2, 1972, prior to the lenses being mounted in the frames. Tests carried out on lenses in frames included the free fall impact of a 5/8 inch diameter steel ball at 50 and 70 inches. The goggles were supported on a fixture specified in ANSI Z 80.7 for testing lens retention under impact. The two sets of lenses when tested, were mounted in identical metal eyeglass frames. Although there is a clear indication that the failure rate of the thinner lenses in metal frames was higher when impacted by the steel ball from 50 inches, the results were pronounced and higher when impacted by the ball from 70 inches. The mean impact resistance measured in terms of inches of free fall height of a 5/8" diameter steel ball causing lens fracture of the thinner lens in metal frames for a given representative sample was 79 inches with a standard deviation of approximately .+-.22 inches whereas the mean impact resistance of the thicker lens in metal frames for a similar representative sample was 101 inches with a standard deviation of approximately .+-.29 inches. The before stated results are representative results from a significant number of tests conducted in this comparison evaluation.